(a) Field
The present application relates to a method of generating a downlink frame and a method of searching for cells. More particularly, the present application relates to a method of generating a downlink frame and a method of searching for cells by using the downlink frame in an orthogonal frequency division multiplexing (OFDM)-based cellular system.
(b) Description of the Related Art
In a direct sequence code division multiple access (DS-CDMA) system, a sequence hopping method is applied to a pilot channel so as to acquire cell synchronization and unique cell identification information. According to the sequence hopping method, a mobile station easily performs a cell search without a separating synchronization channel by introducing a sequence hopping technology to the pilot channel. However, in the OFDM system, a number of channels that are capable of being distinguished by a frequency domain in a symbol duration of one time domain is greater than that of those that are capable of being distinguished by a spread of COMA in the symbol duration of one time domain. Accordingly, when only the time domain is used, resources may be wasted in terms of capacity. For this reason, it is inefficient to directly apply the sequence hopping method to the time domain of the pilot channel in the OFDM-based system. Therefore, it is preferable to search for the cell by efficiently using received signals in both time domain and frequency domain.
An example of an existing technology for searching for a cell in the OFDM system includes a method that allocates synchronization information and cell information by dividing one frame into four time blocks. For the above-described method, two frame structures have been proposed. In a first frame structure, synchronization identification information, cell group identification information, and cell unique identification information are allocated to four time blocks, respectively. In a second frame structure, the synchronization identification information and the cell unique identification information are allocated to a first time block and a third time block, and the synchronization identification information and the cell group identification information are allocated to a second time block and a fourth time block.
According to the first frame structure, since the symbol synchronization is acquired in only the first time block, it is impossible for the mobile station to conduct rapid synchronization acquisition within a prescribed 5 ms during power-on or handover between heterogeneous networks. In addition, it is difficult to acquire diversity gain by accumulating synchronization identification information so as to conduct rapid synchronization acquisition.
According to the second frame structure, the unique cell identification information or the cell group identification information is correlated along with the synchronization acquisition. Therefore, a cell searching process is complex and a rapid cell search is difficult.
As an example of another technology for searching for the cell, a method of acquiring the synchronization and searching for the cell by using a separate preamble has been proposed. However, this method cannot be applied to a system in which the preamble does not exist. Moreover, the preamble is disposed in front of the frame. Accordingly, in a case in which the mobile station would like to acquire the synchronization at a time location that is not the start of the frame, there is a problem in that it must wait for the next frame. Particularly, the mobile station should acquire initial symbol synchronization within 5 msec during the handover among a GSM mode, a WCDMA mode, and a 3GPP LTE mode, but may acquire the synchronization by a frame unit. For this reason, in some cases, the mobile station cannot acquire the initial symbol synchronization within 5 msec.
As an example of another technology for searching for a cell, there is a method of searching for the cell by allocating two short sequences to a secondary synchronization channel and by mapping cell ID information to a combination of two short sequences. According to this method, since interference occurs between cells when the same short sequence is allocated to sectors adjacent to each other, there is a problem in that performance in searching cells is reduced.